Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. If sufficiently severe, cardiac ischemia results in an acute myocardial infarction (AMI), also referred to as a heart attack. With AMI, a substantial portion of heart muscle ceases to function because it no longer receives oxygen, usually due to significant blockage of the coronary artery. Generally, AMI occurs when plaque (such as fat, cholesterol, and calcium) builds up and then ruptures in the coronary artery, allowing a blood clot or thrombus to form. Eventually, the blood clot completely blocks the coronary artery and so heart tissue beyond the blockage no longer receives oxygen and the tissue dies. In many cases, an AMI proves fatal because too much tissue is damaged to allow continued functioning of the heart muscle. Indeed, AMI is a leading cause of death here in the United States and worldwide. In other cases, although the AMI itself is not fatal, it strikes while the victim is engaged in potentially dangerous activities, such as driving vehicles or flying airplanes, and the severe pain and possible loss of consciousness associated with AMI results in fatal accidents. Even if the victim survives the AMI, quality of life may thereafter be severely restricted.
Often AMI is preceded by episodes of cardiac ischemia that are not sufficiently serious to cause actual permanent injury to the heart tissue. Nevertheless, these episodes are often precursors to AMI. Episodes of cardiac ischemia may also trigger certain types of arrhythmias that may prove fatal, particularly ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically, resulting in little or no net flow of blood from the heart to the brain and other organs. Indeed, serious episodes of cardiac ischemia (referred to herein as acute myocardial ischemia) typically result in either a subsequent AMI or VF, often within one to twenty-four four hours, sometimes within only a half an hour or less. Accordingly, it would be highly desirable to provide a technique for reliably detecting cardiac ischemia in real-time so that the victim may be warned and medical attention sought. If properly warned, surgical procedures may be implemented to locate and remove the growing arterial blockage or anti-thrombolytic medications may be administered. At the very least, such warnings would allow the victim to cease activities that might result in a fatal accident. Moreover, in many cases, AMI or VF is triggered by strenuous physical activities and so ischemia warnings would allow the victim to cease such activities, possibly preventing AMP or VF from occurring.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein, or are candidates for such devices. Accordingly, techniques have been developed for detecting cardiac ischemia using implanted medical devices. In particular, techniques have been developed for analyzing intracardiac electrogram (IEGM) signals sensed by such devices in an effort to detect cardiac ischemia. See, for example, U.S. Pat. No. 6,108,577 to Benser, entitled “Method and Apparatus for Detecting Changes in Electrocardiogram Signals.” See, also, U.S. Pat. Nos. 5,113,869 to Nappholz; 5,135,004 to Adams et al.; 5,199,428 to Obel et al.; 5,203,326 to Collins; 5,313,953 to Yomtov et al; 6,501,983 to Natarajan et al.; 6,016,443, 6,233,486, 6,256,538, and 6,264,606 to Ekwall; 6,021,350 to Mathson; 6,112,116 and 6,272,379 to Fischell et al; 6,128,526, 6,115,628 and 6,381,493 to Stadler et al; and. Many IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation of the ST segment of the IEGM that occur during cardiac ischemia. The ST segment represents the portion of the cardiac signal between ventricular depolarization (also referred to as an R-wave or QRS complex) and ventricular repolarization (also referred to as a T-wave). Herein, the ST segment elevation pertains to the amplitude of the ST segment relative to some isoelectric baseline and hence can be positive or negative. Moreover, the elevation can increase or decrease relative to the baseline due to ischemia or other factors. A change in the ST segment elevation (which may also be referred to as an ST segment deviation) is typically measured relative to a historical elevation baseline. Note that the QRS complex usually follows an atrial depolarization (also referred to as a P-wave.) P-waves, R-waves and T-waves are also regarded as features of a surface electrocardiogram (EKG). For convenience and generality, the terms P-wave, T-wave and T-wave are used herein to refer to the corresponding features of internal cardiac signals, i.e. IEGMs.
Typically, with ST-based techniques, the amount of deviation, if any, from a baseline ST segment elevation is compared by the implanted device against a predetermined threshold. If the amount of deviation exceeds the threshold, cardiac ischemia is deemed to have occurred. Warning signals may be generated and, in at least some devices, therapy may be automatically adjusted in response to the ischemia. Often, the threshold is set by the physician during a programming session following device implant but is not otherwise adjusted. Although ST segment elevation is often exploited, other parameters derived from morphological features of the IEGM can instead be used. Other parameters that potentially may be exploited to detect cardiac ischemia include various duration-based parameters such as P-wave width, QRS-complex width and T-wave width; various slope-based parameters such as maximum P-wave slope, maximum QRS-complex slope and maximum T-wave slope; various amplitude-based parameters such as peak P-wave amplitude, peak QRS-complex amplitude and peak T-wave amplitude; as well as various interval-based parameters such as atrioventricular (AV) intervals and the aforementioned ST intervals. Also, devices may exploit the interval between the beginning of a QRS complex and the maximum amplitude (i.e. the peak) of a corresponding T-wave as well as the interval between the beginning of the QRS complex and the end of the corresponding T-wave. These intervals are referred to herein, respectively, as the QTmax interval and the QTend interval. For further discussions regarding various intervals that may be appropriate, alone or in combination with one another, for detecting cardiac ischemia, see U.S. patent application Ser. No. 11/394,724, of Ke et al., filed Mar. 31, 2006, entitled “Ischemia Detection using T-wave Amplitude, QTmax and ST Segment Elevation and Pattern Classification Techniques.” See, also, U.S. Pat. Nos. 7,107,096; 6,985,771; 6,609,023; 6,468,263; 6,272,379; and 6,112,116, each to Fischell et al.
Although the detection of cardiac ischemia is of paramount importance since ischemia may be a precursor to a potentially fatal AMI or VF, it is also desirable to detect hypoglycemia, hyperglycemia, or other abnormal physiological conditions so as to provide suitable warning signals. Diabetic patients, particular, need to frequently monitor blood glucose levels to ensure that the levels remain within acceptable bounds and, for insulin dependent diabetics, to determine the amount of insulin that must be administered. Various techniques have been developed for detecting hypoglycemia and hyperglycemia based on features of electrical cardiac signals, particularly ST segments and T-waves. See, for example, U.S. patent application Ser. No. 11/043,612, of Gill et al., filed Jan. 25, 2005, entitled “System and Method for Distinguishing Among Ischemia, Hypoglycemia and Hyperglycemia Using an Implantable Medical Device.” See, also, U.S. Pat. Nos. 7,272,436 and 7,297,114, to Gill et al., also entitled “System and Method for Distinguishing Among Ischemia, Hypoglycemia and Hyperglycemia Using an Implantable Medical Device.” See, also, U.S. patent application Ser. No. 11/127,370, of Bharmi, filed May 11, 2005, entitled “System and Method for Distinguishing Between Hypoglycemia and Hyperglycemia Using an Implantable Medical Device” (which is a CIP of application Ser. No. 11/043,612) and U.S. patent application Ser. No. 11/117,624, also of Bharmi, filed Apr. 27, 2005, entitled “System and Method for Detecting Hypoglycemia Based on a Paced Depolarization Integral Using an Implantable Medical Device.” A technique for detecting cardiac ischemia based on T-waves is set forth in U.S. Pat. No. 7,225,015, entitled “System and Method for Detecting Cardiac Ischemia Based on T-Waves Using an Implantable Medical Device”, to Min et al. See, also, U.S. patent application Ser. No. 11/740,175, of Fard et al., filed Apr. 25, 2007, entitled “System and Method for Efficiently Distinguishing among Cardiac Ischemia, Hypoglycemia and Hyperglycemia using an Implantable Medical Device and an External System.”
Accordingly, it would be also desirable to provide improved techniques for detecting cardiac ischemia and distinguishing cardiac ischemia from hypoglycemia, hyperglycemia or other systemic influences on IEGM morphology and it is to this end that various aspects of the present invention are directed.